Matrix-type display apparatuses with a memory effect include not only a phase transition liquid crystal display apparatus as disclosed in Japanese Unexamined Patent Application No. 107521/1993 (Tokukaihei 5-107521), but also a ferroelectric liquid crystal display apparatus as disclosed in Japanese Unexamined Patent Application No. 20715/1991 (Tokukaihei 3-20715), a plasma display apparatus as disclosed in Japanese Unexamined Patent Application No. 43829/1994 (Tokukaihei 6-43829), etc.
In general, the matrix-type liquid crystal displays have such characteristics that a selection period is required independently for each scanning electrode, which makes it impossible to select a plurality of scanning electrodes at one time. In each of the described matrix-type display apparatuses, a display is performed by varying a voltage to be applied to the scanning electrode. Specifically, a selection voltage for determining a display state of a pixel is applied, and then a holding voltage for holding the selected display state of the pixel is applied. Lastly, an erase voltage is applied to erase the display state of the pixel. The display state of the pixel can be erased also by stopping the application of the holding voltage.
In the described display apparatuses, a gradation display is enabled, for example, by the scanning method disclosed in Japanese Unexamined Patent Application No. 226178/1988 (Tokukaisho 63-226178). The scanning method will be explained in reference to FIG. 24.
FIG. 24 is a typical depiction of a scanning method of the matrix-type display apparatus including 15 scanning electrodes L.sub.1 -L.sub.15, wherein the scanning electrode L.sub.1 -L.sub.15 are selected in order according to the numbers (1-60) appended to the top line. To respective blocks, numbers "1" through "4" are appended indicative of the bit numbers of respective data to be applied to pixels on the scanning electrodes L.sub.1 through L.sub.15.
In this example, data is applied from the 1st selection period to the 4th selection period in the following manner. In the 1st selection period, data of the 4th bit is applied to the scanning electrode L.sub.15, and in the 2nd selection period, data of the 1st bit is applied to the scanning electrode L.sub.1. In the 3rd selection period, data of the 2nd bit is applied to the scanning electrode L.sub.3, and in the 4th selection period, the data of the 3rd bit is applied to the scanning electrode L.sub.7.
In the described method, a scanning operation can be performed with respect to the described display apparatus with a memory effect by applying an erase voltage and a selection voltage in the selection period.
In the described scanning method, it is assumed that the four selection periods are subjected to selection at the same time. Thus, by applying data in the described order, the ratio of the display period T.sub.1 of the 1st bit, the display period T.sub.2 of the 2nd bit, the display period T.sub.3 of the 3rd bit and the display period T.sub.4 of the 4th bit is selected to be T.sub.1 :T.sub.2 :T.sub.3 :T.sub.4 =1:2:4:8.
On the other hand, in the scanning method disclosed in Japanese Unexamined Patent Application No. 56936/1987 (Tokukaisho 62-56936), the ratio of the display periods is selected to be 1:2:4 by altering the blanking period (application period of a reset pulse).
However, the ratio of respective display periods actually derived from FIG. 24 is T.sub.1 :T.sub.2 :T.sub.3 :T.sub.4 =3:7:15:35. This ratio can be altered depending on which one of the 1st through 4th bits is applied in the 1st selection period. In the described scanning period, it is merely assumed as if a plurality of scanning electrodes were subject to selection at the same time although the plurality of scanning electrodes are, in fact, selected sequentially. Thus, it is not possible to adjust the ratio of the display periods to be exactly 1:2:4:8 (=4:8:16:32).
In the latter example of the scanning method, more than 30 percent of all the selection periods is not related to the brightness, and a sufficient brightness cannot be ensured.